One-third of all pregnant women are obese. Obese pregnant women develop metabolic abnormalities (insulin resistance, hyperglycemia, and hyperlipidemia), and gestational diabetes mellitus (GDM) more frequently than do non-obese women. They also have larger babies more prone to obesity and type 2 diabetes later in life. Low glycemic load diets lower the glycemic and insulin response to meals in nonpregnant adults. We hypothesize that a low glycemic load (GL) diet consumed during the last half of pregnancy lowers the insulin, glucose, free fatty acid response to meals which, in turn, reduces GDM and fetal overgrowth in obese pregnant women, especially those with increased visceral fat. The specific aims are 1) to determine the effects of a low GL diet on insulin, carbohydrate, and fat metabolism in 110 obese (>35% fat) women stratified by visceral fat thickness and randomly assigned to a low or moderately-high GL diet beginning at 20 weeks gestation and continuing to term; 2) to evaluate the effects of low GL diets on the levels of fasting and postprandial hormones involved in regulating fuel metabolism (human chorionic somatomammotropin, progesterone, leptin, adiponectin, prolactin, glucagon, cortisol, catecholamines, and ghrelin); and 3) to determine if consuming low GL diets the last half of pregnancy reduces the prevalence of GDM and fetal macrosomia. At 28 and 34 weeks, the hormonal and substrate response will be measured to a standard 100 g oral glucose tolerance test and to standardized low or moderately-high GL meals; hepatic glucose production and lipolysis will also be measured at 34 weeks when insulin resistance reaches its peak during gestation. The results of our study may provide a new dietary strategy for reducing the risk of gestational diabetes and fetal macrosomia in obese, pregnant women. In addition, new information on how the source of maternal carbohydrate intake affects carbohydrate and lipid metabolism in pregnancy will be generated.